A conventional shadow-mask-type CRT comprises an evacuated envelope having therein a viewing screen comprising an array of phosphor elements of three different emission colors arranged in a cyclic order, means for producing three convergent electron beams directed towards the screen, and a color selection structure or shadow mask comprising a thin multi-apertured sheet of metal precisely disposed between the screen and the beam-producing means. The shadow mask shadows the screen, and the differences in convergence angles permit the transmitted portions of each beam to selectively excite phosphor elements of the desired emission color.
A conventional CRT shadow mask is typically manufactured by first coating a photoresist on a thin metal plate made of Invar or aluminum-killed (AK) steel. The plate is then exposed to light, developed, and etched to form a plurality of holes therein. Thereafter, the plate formed with the holes is annealed using a heat-treating process in a hydrogen atmosphere at a high temperature, thereby removing stress and providing malleability to the plate. The plate is then formed into a predetermined mask shape by a press, after which the plate is cleaned to remove all contaminants from the surface thereof including fingerprints, dust and other foreign substances. Finally, a blackening process is performed on the shaped plate to prevent doming of the same, thereby completing the manufacture of the shadow mask.
The shadow mask acts as a bridge between electron beams emitted from three electron guns (means for producing three convergent electron beams) and red, green and blue phosphor pixels formed on the panel, ensuring that the electron beams land on the correct phosphor pixels. Accordingly, any deviation of the shadow mask from its original position acts to misdirect the electron beams to excite the unintended phosphor pixels.
The shadow mask can be repositioned in the CRT if it receives external shock or vibrations, or as a result of the impact from speakers mounted in the system to which the CRT is applied. That is, if the CRT receives a substantial degree of such forces, the shadow mask moves in the CRT such that electron beams passing therethrough land on the wrong phosphor pixel, thereby deteriorating color purity. This will be described in more detail hereinbelow.
FIG. 1 shows a partial sectional view of a conventional CRT used to describe the shifting of a shadow mask caused by an external shock. As shown in the drawing, the CRT includes a panel 1, a phosphor screen 2 formed on an inner surface of the panel 1, and a shadow mask 6 fixedly suspended a predetermined distance from the phosphor screen 2 and having a plurality of apertures (not shown) formed therein. The shadow mask 6 is mounted to a side wall of the panel 1. That is, a mask frame 5 joined to a periphery of the shadow mask 6 is coupled to a spring 4, and the spring 4 is connected to a stud pin 3 protruding from the side wall of the panel 1. An electron gun 11 is mounted in a funnel (not shown) of the CRT and emits electron beams 10 in a direction toward the shadow mask 6.
When the CRT receives a substantial external shock or vibrations, the shadow mask 6 is shaken and moves from its initial position to a deviated position 7. As a result, the electron beams 10 emitted from the electron gun 11 pass through an incorrect aperture of the shadow mask 6. That is, an electron beam that is intended to pass through a predetermined aperture 8 of the shadow mask 6, comes to pass through an incorrect aperture 9 as a result of the shadow mask 6 moving to the deviated position 7. Accordingly, a position P1 on the phosphor screen 2 on which the electron beam 10 lands is altered to deviated position P2, resulting in the excitation of the wrong phosphor pixel. This causes shaking of the displayed picture, a reduction in color purity and other picture quality problems.
Furthermore, in the case where the CRT receives an extreme shock, for example if the system in which the CRT is installed is dropped, it is possible for the shadow mask 6 to become deformed. An example of this is shown in FIG. 2 in which a deformed area 12 is illustrated. When electron beams 10 pass through the deformed area 12, the above problems of shaking of the displayed picture and a reduction in color purity occur, in addition to the generation of spurious colors.
To remedy the above described problems, Japanese Patent Laid-Open No. Sho 62-223950 discloses a technique for improving tensional strength of the shadow mask by forming a plating layer thereon. However, aperture size is decreased when using this technique.
Also, Japanese Laid-Open Nos. Sho 56-121257 and Hei 1-276542 each disclose a technique for improving tensional strength of the shadow mask by heat-treating the same in a gaseous atmosphere. However, in these conventional methods, the shadow mask is thermally deformed as a result of heat treating the same for long periods during the manufacturing process.